Electro-galvanic gold plating process

ABSTRACT

A gold plating arrangement is described which consists essentially of a plating bath consisting of 0.5 to 10 grams per liter lithium or potassium aurocyanide and 15 to 150 grams per liter ammonium chloride dissolved in water, and a base metal in contact with a second metal which occupies a higher position in the EMF series than gold or said base metal. The resultant plate is much denser than that obtainable by conventional electrolytic plating, thus serving as an effective corrosion protective surface against acid media.

United States Patent [191 Wright et al.

[ Apr. 29, 1975 ELECTRO-GALVANIC GOLD PLATING PROCESS [75] Inventors:Ralph R. Wright, Oak Ridge; Calvin C. Wright, Lenoir City; George S.Petit, Oak Ridge, all of Tenn.

[22] Filed: Mar. 21, 1974 [21] Appl. No.: 453,480

[56] References Cited UNITED STATES PATENTS 5/1957 Pigeon 204 243 6/l968Greene 204/248 X FOREIGN PATENTS OR APPLICATIONS 931,638 7/1963 UnitedKingdom 204/46 G 15,383 1899 United Kingdom 204/249 Primary Examiner-G.L. Kaplan Attorney, Agent, or Firm-Dean E. Carlson; David S. Zachry;Irving Barrack [57] ABSTRACT A gold plating arrangement is describedwhich consists essentially of a plating bath consisting of 0.5 to 10grams per liter lithium or potassium aurocyanide and 15 to 150 grams perliter ammonium chloride dissolved in water, and a base metal in contactwith a second metal which occupies a higher position in the EMF seriesthan gold or said base metal. The resultant plate is much denser thanthat obtainable by conventional electrolytic plating, thus serving as aneffective corrosion protective surface against acid media.

3 Claims, N0 Drawings ELECTRO-GALVANIC GOLD PLATING PROCESS BACKGROUNDOF THE INVENTION The present invention relates to a gold platingprocess. It was made in the course of, or under, a contract with the US.Atomic Energy Commission.

It is a principal object of this invention to provide a novel goldplating bath composition as well as a method for utilizing saidcomposition to achieve a thin, uniform, dense, and adherent gold platedeposited on a selected metal substrate in a method which requires noexternal source of electricity.

Because gold is virtually impervious to chemical attack in highly acidicmedia, it is ideally suitable for use as a protective coating orcladding on less corrosionresistant metals such as copper, brass, monel(coppernickel), nickel, and bronze. Electrolytic plating processes haveroutinely been used to apply a gold layer on metal substrates.Experience has shown, however, that the density of the gold plate isfrequently insufficient to effectively insulate the metal substrate fromcorrosive media. In addition, the throwing power of electrolyticprocesses cannot reach inaccessible areas to guarahtee uniform plating.The same objections apply to the technique of mercurial gilding in whicha gold amalga is painted or brushed on a work surface and then he ted tovolatilize the mercury. A uniform gold plate ca be obtained by theso-called immersion method w ich simply involves immersion ofa basemetal in a suit' ble gold cyanide solution. Plating occurs by virtue ofthe electrochemical action resulting from proximity of two dissimilarmetals, i.e., the plating metal (gold) and the base metal. However, assoon as a thin film of gold is deposited on the base metal, furtherplating stops because there is then no longer any dissimilar metalsurface available to generate the required electrochemiical action.

The present invention is similar to the immersion method in that itrequires no external source of electricity to effect plating. However,it differs in that electrochemical action can be sustained over a longperi d of time to permit deposition of a thin, dense. adher nt gold filmof a desired thickness.

SUMMARY OF THE INVENTION According to the invention, a sustainedelectrogalvanic action is obtained in a selected gold plating solutionbetween a metal workpiece and a metal electrode in contact with saidworkpiece, in which said metal electrode occupies a higher position inthe electromotive series than gold or the metal workpiece, withreference to Lunges Handbook of Chemistry, ninth Edition, p. 1,218, in aselected gold plating solution which has the capacity to dissolve theelectrode metal and maintain its ions in solution. The gold plating bathin which sustained electro-galvanic and hence plating action takes placeto permit practice of the process aspect of this invention is composedof 0.5 to grams/liter lithium or potassium aurocyanide and -150 grams/-liter ammonium chloride.

In practice, a given workpiece to be plated is immersed in the platingsolution in contact with a selected metal electrode. The metal electrodeserves as an anode while the surface of the workpiece serves as acathode. Among metals which may serve as anode are nickel, magnesium,cobalt, iron, zinc, and aluminum. Among metals which may be effectivelyplated with the plating bath are copper, nickel, brass, monel, andsilver. The ammonium chloride dissolves the anode metal and maintainsthe resultant ions in solution, apparently as a chloride complex, over apH range from 4.5 to 6.5. If the pH decreases below 4.5, hydrogencyanide is released. The plating rate drops to almost zero at higher pHvalues.

An effective plating rate for producing an adherent, dense, protectivegold plate occurs at a bath temperature of from 90 to 100 C. with a goldsalt concentration in the range 0.5 to up to its maximum solubility, l0grams/liter. A temperature lower than 90C. or a lower gold saltconcentration leads to impractically low plating rates for producing aneffective corrosion protective plate but is suitable to form anultra-thin plate useful as a decorative finish. When the bathtemperature exceeds 100C., hydrogen pitting of the plate occurs. Forbest results, the solution should be maintained at a temperature of 98C.at a pH of 5.3 at a potassium aurocyanide concentration which does notfall below about 1 gram/liter. During plating, pH may be lowered with aHCl-NH Cl mixture or may be raised with a mixture of NH,OHNH CI, and, asthe gold in solution is consumed, make-up gold should be added. Underthese conditions, plating rates will range from 0.25 to 100 microinchesper hour to deposit a continuous, adherent, dense gold plate on allcathode surfaces of the base metal in contact with the bath. Althoughthe plating rate is low in comparison to rates obtainable withelectrolytic plating, the quality of the plating in terms of density ismuch higher to provide effective protection of the base metal fromchemical attack by corrosive reagents. The quality of the plate in termsof corrosion protection is not a function of plate thickness so much asit is plate density.

The utility and advantages of this invention are illustrated in thefollowing representative example.

EXAMPLE A brass workpiece was immersed in a solution containing 1gram/liter potassium aurocyanide and grams/liter ammonium chloride incontact with a nickel anode, the nickel anode being in a cloth anodebag. Plating was performed at a temperature from 98 to 100C. at a pHranging from 5.3 to 5.5. After 20 hours, a uniform, continuous golddeposit 0.0003 inch thick formed on the workpiece. The plated part wasimmersed in a circulating solution of 15 percent hydrofluoric acid at atemperature of C. After 27 hours no apparent corrosion was noted. A partwhich was electrolytically plated to 0.001 inch thickness and exposed tothe same conditions was found to have been extremely corroded.

What is claimed is:

l. A method for depositing metallic articles with gold which comprisesimmersing said article in a plating bath maintained at a temperature inthe range -100C and at a pH in the range 4.5-6.5, said bath consistingessentially of 0.5 to 10 grams/liter lithium or potassium aurocyanideand 15l50 grams/liter ammonium chloride, while said article is inelectrical contact with a dissimilar metal which renders the articlecathodic in said bath.

2. The method according to claim 1 in'which the pH of the bath ismaintained within the range 4.5-65 by addition of either HCINH,C1mixture or a mixture of NH OH-NH CL 3. The method according to claim 1in which the dissimilar metal occupies a higher position in theelectromotive series than gold or the metal article.

1. A METHOD FOR DEPOSITING METALLIC ARTICLES WITH GOLD WHICH COMPRISINGIMMERSING SAID ARTICLE IN A PLATING BATH MAINTAINED AT A TEMPERATURE INTHE RANGE 90*-100*C AND AT A PH IN THE RANGE 4.5-6.5 SAID BATHCONSISTING ESSENTIALLY OF 0.5 TO 10 GRAMS/LITER LITHIUM OR POTASSIUMAUROCYANIDE AND 15-150 GRAMS/LITER AMMONIUM CHLORIDE, WHILE SAID ARTICLEIS IN ELECTRICAL CONTACT WITH A DISSIMILAR METAL WHICH RENDERS THEARTICLE CATHODIC IN SAID BATH.
 2. The method according to claIm 1 inwhich the pH of the bath is maintained within the range 4.5-6.5 byaddition of either HCl-NH4Cl mixture or a mixture of NH4OH-NH4Cl.
 3. Themethod according to claim 1 in which the dissimilar metal occupies ahigher position in the electromotive series than gold or the metalarticle.